1. Field of the Invention
The present invention relates to an apparatus and a method for mounting a semiconductor device onto a circuit board, and more particularly to a capillary provided for a wire bonding apparatus and a method of forming electric connection contacts (bumps) which electrically connect electrode pads of the semiconductor device to terminal electrodes of the circuit board.
2. Description of the Related Art
Recently, the number of electrode pads per LSI chip has been increased, and the pitch of the connecting terminals has been shortened, thereby making conventional soldering methods difficult to use for mounting a semiconductor device.
In order to solve this problem, recently, improved methods have been proposed in which a semiconductor device such as an LSI chip, is directly mounted on input/output terminal electrodes of a circuit board. Among these improved methods, a flip chip bonding method in which a semiconductor device is mounted facedown on a circuit board is known to be effective because the semiconductor device is electrically connected to a large number of terminal electrodes of the circuit board at one time with the resulting connections having good mechanical strength. Bumps (projecting electrodes) which electrically connect the semiconductor device with electrode pads of the circuit board are formed by electrolytic plating, solder bath immersion, vapor deposition, ball bonding using wire bonding method, and the like.
For example, conventional typical capillaries for wire bonding apparatus are shown in the Bonding Handbook and the Capillary Catalog by MICRO-SWISS Co.
FIG. 19 shows an exemplary conventional capillary 100 for a wire bonding apparatus, and FIG. 20A is a schematic cross-sectional view showing a tip portion 110 of the capillary 100. FIG. 20B shows the shape of the tip portion 110 of the capillary 100 in more detail.
As is shown in FIG. 20A, in the cylindrical capillary 100, there is provided a bore 102 into which a metal wire for bonding is inserted. The diameter of the bore 102 is in the range of about 25 .mu.m to 50 .mu.m. The tip portion 110 of the capillary 100 has a cone shape with an angle .alpha. of about 30.degree., in view of the bonding pitch, and the shape and size of the metal wire (or a bump) after the bonding.
FIG. 20B, for example, shows the shape of the tip portion 110 of the capillary 100 when a metal wire having a diameter of about 25 .mu.m is used. In this case, the bore diameter (hole diameter) H is 38 .mu.m, the tip diameter T is 203 .mu.m, and the chamfer diameter CD is 74 .mu.m. By using a capillary 100 having such a shape, it is possible to form bumps on electrode pads of a semiconductor device and to realize the electric connection to terminal electrodes of circuit board. The capillary shown in FIGS. 20A and 20B is usually used for the bonding with a bonding pitch of 140 .mu.m or more.
FIGS. 21, 22A, and 22B show another exemplary conventional capillary 200 for a wire bonding apparatus. The capillary 200 is usually used for the bonding with a bonding pitch less than 140 .mu.m. As is shown in FIG. 21, the end portion of the cylindrical capillary 200 has a bottleneck 210 for the narrower bonding pitch. The height NH of the bottleneck is usually about 500 .mu.m. In the example shown in FIG. 22A, the height NH is 460 .mu.m. In the capillary 200, there is provided a bore 102 into which a metal wire for bonding is inserted, the same as in the capillary 100. The diameter of the bore 102 is in the range of about 25 .mu.m to 50 m. The bottleneck 210 of the capillary 200 has a cone shape with an angle .beta. of about 10.degree., in view of the bonding pitch and the shape and size of the metal wire (or a bump) after the bonding.
FIG. 22B, for example, shows the shape of the end portion of the capillary 200 when a metal wire having a diameter of about 25 .mu.m is used. In this case, the bore diameter (hole diameter) H is 38 .mu.m, the tip diameter T is 152 .mu.m, and the chamfer diameter CD is 64 .mu.m. By using a capillary 200 having such a shape, it is possible to form bumps on electrode pads of a semiconductor device and to realize the electric connection to terminal electrodes of circuit board.
Next, a conventional method of forming bumps of a semiconductor device by ball bonding will be described using the above-described capillary for wire bonding. For example, Japanese Laid-Open Patent Publication No. 2-34949 shows electric contact bumps each having a two-stage projecting shape (hereinafter, referred to as two-stage bumps), and a method of forming such two-stage bumps using a conventional capillary.
FIGS. 23A to 23D schematically show a method of forming a two-stage bump 107 on an electrode pad 103 which is formed on an IC chip 106, using a capillary 101 by conventional ball bonding.
First, as is shown in FIG. 23A, a metal wire 104 having a diameter of 25 .mu.m is inserted into a bore 102 of the capillary 101. Thermal energy is applied to the end of the metal wire 104 by a gas flame, electric pulses, ultrasonic vibration, or the like. As a result, a ball 105 having a diameter which is about two or three times as large as the diameter of the metal wire 104 is formed at the end of the metal wire 104.
Next, as is shown in FIG. 23B, the ball 105 formed at the end of the metal wire 104 is caused to abut against the electrode pad 103 of the IC chip 106 by moving the capillary 101 downwards. By thermo-compression bonding, or by the application of ultrasonic vibration, the ball 105 is secured to the electrode pad 103, so as to form a pedestal portion 109 of the bump 107 (the first bonding step). The pedestal portion 109 has an outer diameter of about 80 .mu.m to 90 .mu.m, and a height of about 20 .mu.m to 30 .mu.m.
Then, as is shown in FIG. 23C, under the condition that the pedestal portion 109 of the bump 107 and the metal wire 104 inserted in the bore 102 of the capillary 101 are connected, the capillary 101 is moved in a loop. Specifically, the capillary 101 is first moved upwards above the pedestal portion 109 of the bump 107, and then moved in a loop. Thereafter, while the capillary 101 is moved downwards, the metal wire 104 is cut (the second bonding, see FIG. 23D). As is shown in FIG. 23D, by the loop movement of the capillary 101, a ring-shaped or reversed U-shaped portion of the metal wire 104 is formed on the pedestal portion 109. The portion constitutes a top portion 108 of the bump 107. The metal wire 104 is cut by the edge 111 of the tip of the capillary 101. Thus, the formation of the two-stage bump 107 shown in FIG. 24 is completed.
Each of the pressures for the first bonding and the second bonding is set in the range of 20 g to 45 g per one bump, depending on the material of the wire and the wire diameter.
FIG. 24 shows the shape of a typical two-stage bump formed by conventional ball bonding. The two-stage bump 107 (stud bump) has an outer diameter R of about 80 .mu.m to 90 .mu.m, and a total height h.sub.1 of about 60 .mu.m to 80 .mu.m.
In order to make the heights of the two-stage bumps 107 uniform, a leveling process in which the bumps 107 are pressed against a smooth plane is performed (see FIG. 25). The reason why the leveling process is required is that the heights of the bumps formed by ball bonding are largely different from each other, as compared with the heights of bumps formed by other methods. In cases where conductive adhesive is used as an adhesion layer for connecting a semiconductor device to a circuit board, the leveling process can attain another effect in that the transfer amount of the conductive adhesive is stabilized.
As is shown in FIG. 25, in the leveling process, the IC chip 106 is disposed facedown and pressed against the smooth plane 112. The leveling load is generally about 50 g per one bump. The leveling load is adjusted depending on the material of the wire and the wire diameter. FIG. 26 shows a typical shape of the two-stage bump 107 after the leveling process. As the result of the leveling, the total heights h.sub.1 of the two-stage bumps 107 are uniformly set in the range of 40 .mu.m to 50 .mu.m.
The conventional capillary and the conventional bump forming method as described above require a process for forming bumps on a semiconductor device and another process for leveling the formed bumps, which causes high production costs. Moreover, an apparatus for the leveling is additionally required.
However, the omission of the leveling process causes the following problems; first, the top portion 108 of the bump formed by ball bonding (before leveling process) is ring-shaped or reversed U-shaped, so that the top face 113 of the top portion 108 has a small area (see FIG. 24). This means that the contact area with a terminal electrode of the circuit board is disadvantageously small. In addition, the heights of respective bumps are usually varied, so that the bumps without leveling cannot realize the connection with the desired high reliability. Secondly, in the case where the conductive adhesive is used as the adhesion layer, due to the shape of the bumps before the leveling, only small amounts of conductive adhesive can be transferred to the top ends of the bumps, and the transferred amounts are largely varied. Accordingly, the adhesion strength after the curing of the conductive adhesive is low, and hence the adhesion reliability is low. Further, the connection resistance is increased. For these reasons, it is preferred not to omit the leveling process.
Furthermore, by the conventional bump forming method as described above, the bumps may be formed into other shapes as shown in FIGS. 27A to 27C rather than the typical shape shown in FIG. 24. The shapes of the bumps shown in FIGS. 27A and 27B are referred to as "second peeling". That is, when the metal wire 104 is cut by the capillary 101, the end 114 of the ring-shaped or reverse U-shaped portion is peeled from the body of the bump 107. The shape of the bump shown in FIG. 27C is referred to as "tail standing". If a projecting portion of the tail 115 measured from the side of the bump is larger than the quarter of the diameter of the bump pedestal portion, the bump is defective. Such a defective bump occurs when the metal wire 104 is not cut well at a predetermined position by the edge 111 of the capillary 101. The bumps having these shapes are undesirable as connecting contacts. In the bump shown in FIG. 27A, the top portion 116 is separated into two portions, so that sufficient strength cannot be obtained. In the bump shown in FIG. 27C, the tail 115 is projected sideways. If the conductive adhesive is held at the tail 115 and then spread, the risk of the occurrence of a short-circuit with adjacent bumps and electrodes becomes very high. As described above, the bumps of these undesired shapes may cause connection failure and a short-circuit, so that it becomes difficult to connect the semiconductor device to the circuit board with high reliability by using such bumps.
FIGS. 28A and 28B show shapes of other kinds of defective bumps, which are caused by the conventional leveling process. In these bumps, the top portions thereof are undesirably fallen sidewards as the result of the leveling.